Refactor and optimize code for UNPACK_SEQUENCE.
Refactor and optimize code for UNPACK_SEQUENCE.

* Defer error handling for wrong number of arguments to the
  unpack_iterable() function.  Cuts the code size almost in half.

* Replace function calls to PyList_Size() and PyTuple_Size() with
  their smaller and faster macro counterparts.

* Move the constant structure references outside of the inner loops.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator